Liquid crystal display devices used in a variety of fields from small-sized devices such as mobile phones to large-sized devices such as TV sets of 30 inches or larger are each comprised of a thin-film transistor (which will hereinafter be called “TFT”) as a switching device, a transparent pixel electrode, a wiring portion such as gate wiring and source-drain wiring, a TFT substrate equipped with a semiconductor layer such as amorphous silicon (a-Si) or polycrystalline silicon (p-Si), a counter substrate placed to face with the TFT substrate with a predetermined space and equipped with a common electrode, and a liquid crystal layer filled between the TFT substrate and the counter substrate.
In the TFT substrate, Al alloys such as pure Al and Al—Nd (which alloys may hereinafter be called “Al-based alloys”, collectively) have been used widely as a material for gate wiring or source-drain wiring because they have a low electrical resistance and can be easily microfabricated. It is the common practice to place a barrier metal layer made of a refractory metal such as Mo, Cr, Ti, or W between the Al-based alloy wiring and a transparent pixel electrode. The Al-based alloy wirings are thus connected to each other via a barrier metal layer because direct connection between the Al-based alloy wiring and the transparent pixel electrode may raise a contact resistance and deteriorate the visual quality of the display. Described specifically, Al constituting a wiring directly connected to the transparent pixel electrode is considerably susceptible to oxidation. Due to oxygen generated during the film formation procedures of a liquid crystal display or oxygen added upon film formation, an insulating layer of an Al oxide may inevitably be formed on the interface between the Al-based alloy wiring and the transparent pixel electrode. In addition, a transparent conductive film such as ITO constituting the transparent pixel electrode is made of a conductive metal oxide but the Al oxide layer formed as described above prevents an electrical ohmic contact.
Formation of a barrier metal layer, however, requires, in addition to a sputtering apparatus necessary for the formation of a gate electrode, a source electrode, and further a drain electrode, a deposition chamber of a barrier metal. As the cost of liquid crystal displays is decreasing through mass production, it has been impossible to neglect an increase in the production cost and a reduction in productivity attributable to the formation of a barrier metal layer.
There are therefore proposed an electrode material or a manufacturing method that can omit the formation of a barrier meal layer and thereby enables direct connection of an Al-based alloy interconnect to a transparent pixel electrode.
For example, the present applicants disclose a direct contact technique capable of omitting a barrier metal layer and at the same time, directly and reliably connecting an Al-based alloy interconnect to a transparent pixel electrode without increasing the number of steps (Patent Document 1).
More specifically, Patent Document 1 discloses an Al alloy containing, as an alloy component, at least one element selected from the group consisting of Au, Ag, Zn, Cu, Ni, Sr, Ge, Sm, and Bi in an amount of 0.1 to 6 atomic percent. When an Al-based alloy wiring made of the above Al alloy is used, it becomes possible to reduce a contact resistance with a transparent pixel electrode even if a barrier metal layer is omitted, by causing at least a portion of these alloy components to exist as a precipitate or concentrated layer on the interface between the Al alloy film and the transparent pixel electrode.
The Al alloys containing Ni or the like which are described in Patent Document 1 each have a heat-resistant temperature of approximately from 150 to 200° C. and is lower than the maximum temperature in the manufacturing steps of a display device (particularly, a TFT substrate).
The manufacturing temperature of a display device tends to decrease in recent years from the standpoint of improving the yield and productivity. Even if the maximum temperature (deposition temperature of a silicon nitride film) in the manufacturing steps is reduced to 300° C., it exceeds the heat resistant temperature of the Al alloy described in Patent Document 1.
On the other hand, a reduction in the maximum temperature (which will be called “heat treatment temperature” in the present invention) in the manufacturing steps may cause a trouble that the electrical resistance of an Al-based alloy wiring does not decrease adequately. The present applicants therefore disclose, in Patent Document 2, an Al alloy showing an adequately low electrical resistance even at a low heat treatment temperature while showing a good heat resistance.
Described specifically, the present applicants disclose an Al alloy film made of an Al-α-X alloy containing at least one element (which will hereinafter be called “α component”) selected from the group consisting of Ni, Ag, Zn, Cu, and Ge and at least one element (which will hereinafter be called “X component) selected from the group consisting of Mg, Cr, Mn, Ru, Rh, Pd, Ir, Pt, La, Ce, Pr, Gd, Tb, Sm, Eu, Ho, Er, Tm, Yb, Lu, and Dy.
According to this document, when the above Al alloy film is used for a thin-film transistor substrate, a barrier metal layer can be omitted and at the same time, the Al alloy film can be directly and reliably brought into contact with a transparent pixel electrode made of an Al alloy film and a conductive oxide film without increasing the number of the steps. In addition, even when a heat treatment temperature as low as about 100° C. or more but not more than 300° C. is applied to the Al alloy film, both a reduction in electrical resistance and excellent heat resistance can be achieved. More specifically, it is described that even if heat treatment is conducted, for example, at as low as 250° C. for 30 minutes, the Al alloy film can achieve an electrical resistivity of 7 μΩ·cm or less without causing defects such as hillocks.